I think the author is also severely underestimating how early Tolkien starting working on his maps. The Hobbit (published in 1937) already came with a somewhat fleshed out map of the Misty Mountains and surroundings, and it is fair to assume that from this the remainder of Middle-Earth was fleshed out. (I cannot quickly find somewhere when he first drew a complete map of Middle-Earth, but Wikipedia notes that "The paper became soft, torn and yellowed through intensive use, and a fold down the centre had to be mended using parcel tape" [1]. So he probably knew what Middle-Earth looked like for quite some time, maybe even late 1930s?
To presume that Tolkien would have had an understanding of the then state-of-the-art theories around continental drift is a bit hopeful. In fact, plate tectonics did come to be accepted until the 50s/60s, so Tolkien could barely have known of the theory when writing LOTR and especially not when drawing the initial map.
[1] https://en.wikipedia.org/wiki/Tolkien%27s_maps#The_Lord_of_t...